US7132683B1

US7132683B1 - Dual purpose test structure for gate-body current measurement in PD/SOI and for direct extraction of physical gate length in scaled CMOS technologies

Info

Publication number: US7132683B1
Application number: US10/838,230
Authority: US
Inventors: Srinath Krishnan; William George En
Original assignee: Advanced Micro Devices Inc
Current assignee: Advanced Micro Devices Inc
Priority date: 2004-05-05
Filing date: 2004-05-05
Publication date: 2006-11-07

Abstract

A structure, for testing relative to an MOS transistor, closely resembles the MOS transistor of interest. For example, certain dimensions and a number of dopant concentrations typically are substantially the same in the test structure as found in corresponding elements of the MOS transistor of interest. However, the regions of the test structure corresponding to the source and drain of the transistor have no halos or extensions that might cause gate overlap; and in the test structure, these regions are of a semiconductor type opposite the type found in the source and drain of the transistor. The test structure enables accurate measurement of the gate-body current, for modeling floating body effects and/or for direct electrical measurement of gate length.

Description

FIELD OF THE INVENTION

The present subject matter relates to a test structure, typically manufactured in association with a semiconductor device of interest, which can be measured to determine significant operational and/or structural parameters of the semiconductor device.

BACKGROUND

Metal Oxide Semiconductor (MOS) type transistors are a fundamental building block within integrated circuits. Consequently, there is a persistent push to make such devices smaller, faster, etc. An MOS transistor, includes a body region of a first semiconductor type (e.g. a P-type region). The body region has two spaced regions (source and drain) of the opposite semiconductor type (e.g. N-type). Typically, the source and drain regions have lightly doped extension regions of the same semiconductor type (N-type, in our example) although the doping concentration is lower. The region of the body between the source and drain is referred to as the “channel.” The extension regions help overcome short channel transistor effects as device dimensions continue to shrink. A doped polysilicon gate overlies a thin gate oxide. The gate and oxide overly the channel region in the body, between the source and drain.

The silicon-on-insulator (SOI) type MOS structure was developed to improve performance, for example by reducing junction capacitances. This technology involves forming the MOS transistor on an insulating region. The SOI transistor has components similar to the MOS transistor device formed on a bulk semiconductor substrate as described above, however, the body overlies an insulating layer, such as silicon dioxide (SiO₂). The insulating layer, in turn, overlies a bulk semiconductor material. Conventional SOI types of structures have evolved and basically comprise a substrate, such as a silicon-containing substrate, an insulating layer thereon, commonly referred to as a buried oxide layer, and a monocrystalline silicon layer on the insulating layer which constitutes the “body” of the transistor.

An SOI based transistor structure provides several performance advantages over traditional bulk transistor devices. For example, each device in an SOI structure is completely isolated from all other devices (as opposed to sharing a common substrate body). Consequently, the SOI provides better individual device isolation, which prevents circuit latch-up conditions. Also, in most SOI devices, at least a portion of each source or drain region abuts the underlying insulating layer. As a result, the cross sectional area of the source/drain interfaces to the semiconductor body are reduced, and this reduces the junction capacitances. When an electrical signal changes on either or both source/drain regions, there is no capacitive coupling to the substrate. Certain electrical elements of the circuit can be positioned closer together, thereby reducing the die size. SOI structures offer the advantages of latch-up immunity, reduced junction leakage currents and reduced short channel effects, thereby translating to increased transistor speed.

In SOI devices, the body floats in that there is no direct electrical connection thereto. In some cases, this can be disadvantageous. “Floating body effects” are a class of hysteresis effects produced because the voltage of the semiconductor body is allowed to float relative to ground. Examples of floating body effects include the “kink” effect and the parasitic lateral bipolar effect. The “kink” effect originates from impact ionization. When the SOI transistor is operated at a relatively high drain-to-source voltage, channel electrons having sufficient kinetic energy cause an ionizing collision with the lattice, resulting in carrier multiplication near the drain end of the channel. The generated holes build up in the body of the device, thereby raising the body potential. The increased body potential reduces the threshold voltage of the transistor, thus increasing the transistor current, which results in a “kink” in the transistor current/voltage curves. If impact ionization generates a large number of holes, the body bias may be raised to a sufficient voltage so that the source/body p-n junction becomes forward biased. When this junction becomes forward biased, minority carriers are emitted into the body, which causes a parasitic lateral npn bipolar transistor to turn on. This parasitic lateral bipolar transistor effect leads to a loss of gate control of the transistor current and is therefore highly undesirable.

The flow of current between the gate and the body is a relevant parameter having an impact on the above-noted floating body effects. To assess such effects, it is useful to measure the gate-body current of an SOI transistor. However, existing techniques involve making a body tie or connection to the body of the actual device (that otherwise would be floating) and measuring current flow between the gate and the tied body. Characterization or measurement of this component in this manner is difficult, since tied-body structures used to extract the measure of the gate-body current introduce a large error due to the body tie connection to the device. Essentially, the extrinsic gate-body current in a device that has a tie or contact to the body for the measurement overwhelms the intrinsic gate-body current produced solely by the device operation without the tie, due to the topology/behavior of the body tie. Often, the extrinsic gate-body current due to the body tie is 10 to 100 times larger than the intrinsic gate-body current.

In developing and scaling MOS technologies, it is also useful to know the physical gate length. However, there currently is no convenient technique for directly measuring the physical gate length of an MOS transistor, in SOI or bulk technologies.

Hence, there is a need for a test structure and a testing technique, which enable the desirable testing of gate-body current and physical gate length.

SUMMARY OF THE INVENTION

Applicants have developed a test structure, to facilitate test measurements that accurately characterize aspects of an MOS transistor. Although separate from the MOS device, the test structure closely mimics the MOS device. The MOS device and the test structure may be elements constructed on a bulk semiconductor substrate, however, the test structure is particularly advantageous when applied to analysis of transistors in silicon-on-insulator (SOI) type integrated circuits.

The test structure has a device body, of a semiconductor type corresponding to a semiconductor type in a device body of the transistor. For example, if intended for testing in relation to an NMOS transistor, the test structure has a P-type body. Conversely, if intended for testing in relation to a PMOS transistor, the test structure has a N-type body.

The test structure also has a source region and a drain region separated from each other by a portion of the test structure device body forming a channel. The test structure source region and the test structure drain region, however, do not actually function as the source and drain of a transistor, because these regions are formed of a semiconductor type opposite to the semiconductor type used in the source and drain of the transistor. For example, if intended for testing in relation to an NMOS transistor, the test structure has a P-type body and P-type source/drain emulation regions. Conversely, if intended for testing in relation to a PMOS transistor, the test structure has a N-type body and N-type source/drain emulation regions. However, the dopant concentrations in the test structure source region and the test structure drain region substantially correspond to dopant concentrations in the source and the drain of the transistor.

The test structure also has a gate insulator over the channel and a gate over the gate insulator. The test structure gate is substantially free of overlap with either of the test structure source region and the test structure drain region. For example, a typical MOS transistor will have halos or lightly doped extensions of the source and drain, which extend under the gate of the transistor. During manufacture of the test structure, however, care is taken to avoid halo effects and to prevent doping corresponding to the source/drain extensions, that is to say to prevent gate overlap with the regions of the test structure that emulate the source and the drain.

When used to analyze parameters of a particular transistor, for example, the dimensions and dopant concentrations of elements of the test structure closely conform to those of the corresponding elements of the transistor of interest. It is also contemplated that a test structure having generally similar properties but a larger gate/channel area may be constructed for calibration purposes.

In disclosed examples, the test structure and the metal oxide semiconductor transistor are both elements of one integrated circuit. Detailed examples are given of a manufacturing technique enabling construction of the transistor and the test structure, with many common processing steps, and with a minimum number of steps added for assembly of the test structure.

The test structure has certain properties that are quite similar to those of the transistor of interest. For example, the gate is substantially similar in at least one dimension and in resistivity (due to similar dopant concentration), to the gate of the transistor of interest. The channel region and body have similar properties, although in the examples, the test structure does not have any source/drain extensions to shorten the channel or create overlaps with the gate. Overlap prevents scaling of gate current to gate or channel length. Since the examples have no overlap, the test structure exhibits a substantially linear relationship of gate current to gate/channel length. A difference in length results in a corresponding difference in current.

The test structure physically resembles an MOS transistor, but because of the use of different semiconductor type dopant in the regions corresponding to the source and drain, the test structure essentially functions as an MOS capacitor. The source/drain regions, however, enable a connection to the body of the structure, which can be used to measure a gate-body tunneling current. The measured current will be substantially similar to the current in the transistor of interest during actual operation of the transistor. Hence, it is possible to measure and derive information from the gate-body current of the test structure, without the need to provide a body tie or contact to the body of an actual transistor.

A method also is disclosed for testing to analyze one or more characteristics of a metal oxide semiconductor transistor. The method involves constructing a test structure having a body, a gate, a source and a drain similar to those of the transistor. The body of the test structure is the same semiconductor type as in the body of the transistor. However, the source and drain of the test structure are opposite in semiconductor type compared to the source and drain of the transistor. The test method involves measuring current flow through the gate of the test structure and either the source of the test structure or the drain of the test structure.

An exemplary application of the test current measurement involves use of the measurement as a representation of gate-body tunneling current of the transistor, for example for modeling of floating-body effects if the transistor is an SOI device. Another exemplary application of the test current measurement relates to determination of gate length. In this second application, the measured current is divided by a normalized value of gate current per unit length, to determine gate length of the test structure and thus the gate length for the transistor.

Additional objects, advantages and novel features of the examples will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following and the accompanying drawings or may be learned by production or operation of the examples. The objects and advantages of the inventive concepts may be realized and attained by means of the methodologies, instrumentalities and combinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawing figures depict one or more implementations in accord with the present concepts, by way of example only, not by way of limitations. In the figures, like reference numerals refer to the same or similar elements.

FIG. 1 a cross-sectional view of a simple example of a test structure.

FIG. 2 is a simplified flow chart diagram useful in explaining steps of test measurements utilizing the structure of FIG. 1.

FIG. 3 is a simplified flow chart diagram useful in providing a high-level explanation of the processing steps involved in manufacturing the test structure.

FIG. 4 is a cross-sectional view of a base structure for a silicon-on-insulator (SOI) type semiconductor device, showing two active device regions.

FIG. 5 is a cross-sectional view of the SOI structure showing formation of a gate oxide layer.

FIG. 6 is a cross-sectional view of the SOI structure showing implantation of P-Well dopants.

FIG. 7 is a cross-sectional view of the SOI structure showing the N-channel threshold implant step.

FIG. 8 is a cross-sectional view illustrating formation of a polysilicon layer.

FIG. 9 is a cross-sectional view illustrating implantation of dopants and subsequent annealing of the polysilicon layer.

FIG. 10 is a cross-sectional view showing implantation of additional dopants in the portion of the polysilicon layer over the region that will form the test structure.

FIG. 11 is a cross-sectional view showing the gate patterning step.

FIG. 12 is a cross-sectional view illustrating implantation of dopants for formation of source/drain extension regions for the NMOS device.

FIG. 13 is a cross-sectional view showing formation of gate sidewall spacers.

FIG. 14 is a cross-sectional view illustrating deep implantation of dopants for formation of source/drain regions for the NMOS device.

FIG. 15 is a cross-sectional view illustrating deep implantation of dopants of an opposite type for formation of regions of the test structure emulating source/drain regions of the NMOS device.

FIG. 16 is a cross-sectional view showing the NMOS device and the test structure, after removal of masks used in the preceding step.

FIG. 17 is a cross-sectional view showing deposition of a metal layer for silicide formation.

FIG. 18 is a cross-sectional view of showing treatment to form silicide layers within the NMOS device and the test structure.

FIG. 19 is a cross-sectional view of SOI integrated circuit showing the NMOS device and the test structure, after removal of the metal layer used for silicide formation.

FIG. 20 a cross-sectional view of SOI device showing the NMOS transistor and the test structure, useful in explaining measurements conducted on the test structure.

DETAILED DESCRIPTION OF THE EXAMPLES

The disclosure herein relates to examples of a test structure, as well as applications thereof for measuring gate-body currents in MOS transistor devices and/or measuring gate current as a means to measure physical gate length of a transistor device. The test structure is designed to mimic an MOS device of interest for testing and analysis purposes, so that tests on the test structure provide desired information regarding the MOS device of interest.

The MOS device and the test structure may be elements constructed on a bulk semiconductor substrate, however, the test structure and methods of manufacturing the structure are particularly advantageous when applied in the context of silicon-on-insulator (SOI) type integrated circuits. Hence, the discussion of the examples will focus mainly on SOI type implementations.

FIG. 1 illustrates an exemplary test structure 1 implemented as an SOI type device. As shown, the SOI test structure 1 includes a substrate 11, an insulator 13 on the substrate 11, and a semiconductor layer 15 on the insulator 13. In the SOI structure 1, isolation barriers 17 effectively divide the semiconductor layer 15 into one or more isolated regions that form the active device wells. In the test structure 1, the active device well is doped to form a body region 19 of a first semiconductor type. The example is a test structure 1 intended to mimic an NMOS transistor device, therefore the body region 19 is a P-type semiconductor. In preferred examples, the active device well is doped with opposite type materials (e.g. N-type in the NMOS example) to form a channel threshold implant 21. The body 19 and the threshold implant 21 will typically use the same type and substantially the same concentration of dopant as found in similar elements in the transistor of interest.

For test purposes, the

regions

23 and 25 of the structure 1 are intended to represent the source and drain of the transistor of interest. Unlike the typical transistor, however, the

regions

23 and 25 have no halos or extensions that might cause gate overlap. Also, in the test structure 1, these

regions

23 and 25 are of a semiconductor type opposite the type found in the source and drain of the transistor. Hence, in the NMOS example, the test source 23 and the test drain 25 are P+ regions (as opposed to N+ source/drain regions in the NMOS transistor of interest). The level of dopant concentration in the

regions

23 and 25 is substantially the same as that of the dopant (opposite type) in the source/drain regions in the NMOS transistor of interest.

Like a transistor, the test structure 1 also includes a gate 27 on a gate insulator 29. The structure also includes sidewall spacers 31 formed of an insulating material on the sides of the gate 27 and the insulator 29. As noted, the gate 27 does not overlap the source 23 or the drain 25 of the test structure 1. In the example, the test structure gate 27 is separated from overlap with the test structure source region 23 and the test structure drain region 25 by the width of the spacers 31.

The gate 27 and the insulator 29 may have any desired dimensions. For example, for calibration purposes, the structure may be designed as a relatively large area (large channel length) device. However, when intended for test analysis in relation to a particular transistor of interest, the gate 27 as well as the body 19 and the test source/

drain regions

23 and 25 all have dimensions similar to those of the corresponding elements in the transistor of interest. Where formed of doped polysilicon, the concentration of implanted dopant and thus the resistivity of the gate 27 are substantially the same as in the gate of the transistor of interest.

The test structure 1 looks like an MOS type field effect transistor (MOSFET). The illustrated test structure, however, does not include the NPN (or PNP) type regions normally found in a transistor. Essentially, the test structure 1 instead comprises a gate 27, an insulator 29 and a semiconductor well having a number of different regions (19, 23 and 25) having different dopant levels. As such, the test structure 1 is a form of integrated circuit type capacitor. However, the characteristics of the test structure 1 facilitate testing thereof to provide an accurate indication of gate current as it would appear in a corresponding transistor device.

In an SOI transistor, the body floats relative to ground in that there is no specific potential or voltage value applied to the body of such a transistor. In fact, a normal operational SOI transistor does not have any electrical connection to the body portion of the device. However, the P+ ‘source/drain’

emulation regions

23 and 25 in the test structure 1 provide a means to electrically connect to the body in the test structure, for example, to measure current flow through the body.

Measurements on the test structure have at least two significant applications of interest. However, both rely on a measurement of the gate current. Measurements of gate current essentially involve applying a voltage 51 across the gate 27 and one of the body contacts provided by the

regions

23 and 25. The example shows the voltage V connected across the gate 27 and the test source region 23. The voltage 51 applied to the gate may be essentially similar to a maximum voltage level expected to be applied to the gate of the transistor of interest, during normal operation of the transistor. Usually, a curve is measured from minimum to maximum voltage. An appropriate current meter 53, connected in series in the circuit, provides a reading of the gate current flowing through the body of the test structure 1.

Gate-body current (I_GB) is known to impact floating body effects in SOI devices. As noted in the background, direct analysis of the gate-body tunneling current in actual devices is difficult, since tied-body structures used to extract a measure of that current introduce a large error due to the body tie connection to the device. However, measurements taken on the test structure 1 can provide representative data regarding the intrinsic gate-body tunneling current that would actually be present in the corresponding NMOS device of interest. Knowledge of the gate-body tunneling current, from measurements on the test structure 1, are useful in modeling of the current for characterizing floating body effects in circuits.

The test structure 1 also is applicable to measurement of gate current, as a technique to determine the gate length (and effective channel length). Essentially, the gate current is proportional to the gate length in the test structure 1. In the conventional MOSFET, the overlap of the gate over the extensions of the source and drain regions skews the relationship of current to gate length. However, the test structure 1 lacks such gate overlap. By using the test structure built to have gate dimensions that correspond to those of the gate of the MOS transistor device, it is possible to directly extract the physical gate length of interest by measuring current in the test structure 1.

FIG. 2 is a flow-chart useful in explaining a test operation to determine one or more characteristics of an MOS transistor device of interest using the test structure 1. Essentially, the test structure 1 is constructed to closely correspond to the transistor of interest (step 61), for example using techniques discussed in more detail, later. For a current measurement for analysis of floating body effects, the test involves measuring the gate body current (I_G(test)) of the test structure 1 (shown at 63) and output for further processing as the gate-body tunneling current (at 65).

If the test will provide a gate length measurement, then certain calibration steps are necessary. The calibration uses a wide are a version of the test structure (formed at 67), for example, a test structure having an area around 100 μm²(W×L=10 μm×10 μm). The width W for the wide area device should be the same as that in the test structure 1 (and the actual device of interest). The various dopant concentrations also should be similar to those in the test structure (and the actual device of interest). The current through the gate (I_G(cal)) is measured at 69 on the large area version of the test structure and normalized at 71. The voltages applied to the large area test structure during measurement step 69, in the calibration step 69, are the same as in the measurements 61 on the test structure 1. Based on the calibration current measurement on the large area device, the normalization (at 71) provides a value for gate current per unit gate length (normalized value V), that is to say V=I_G(cal)÷L(cal), where V is the normalized value (units of current/μm²).

The ratio of the measured gate current from the test structure 1 (I_G(test)) to the normalized current per unit length V equals the number of units of length of the gate in the test structure 1. Thus, step 73 involves computation of L(test) μm=I_G(test)÷V, which is the value output at 75. Since the test structure 1 has gate dimensions corresponding to those of the NMOS device of interest, the gate length (L(test)) derived from the current measurement substantially equals the gate length of the actual NMOS device.

Those skilled in the art will recognize that the test structure (either as an actual test device or a calibration device) may be produced by known semiconductor processing techniques, for bulk semiconductor integrated circuits or for SOI integrated circuits. As noted, the structure has particular advantages in the SOI context. Hence, to insure a complete understanding, it may be helpful to consider examples of applicable SOI processing techniques. An exemplary manufacturing process is described, first at a high level with regard to FIG. 3 and then in more detail with reference to an example shown in FIGS. 4–19.

Although the present SOI manufacturing technique is described in the context of fabricating an NMOS type device and a corresponding test structure, those skilled in the art will recognize that the technique is readily adaptable to production of a PMOS type device and a corresponding test structure. It will be appreciated that the technique is applicable to a variety of other types of transistor devices. The present description will enable those skilled in the art to practice the manufacturing technique to produce test structures associated with a vast number of different types of transistor devices.

FIG. 3 is a simplified flow chart outlining the processing steps involved in manufacturing the test structure. As shown, the method involves forming an active device well (step S1), for example, in a silicon-on-insulator structure including a substrate 111, an insulator 113 on the substrate 111, and a semiconductor layer 115 on the insulator 113 (see FIG. 4). In the SOI example, the active device well 121 comprises an isolated region of the semiconductor layer, for example, separated from other wells 119 by isolation barriers 117.

The method (FIG. 3) also involves a step S2 of implanting a dopant 125 of a first semiconductor type in the active device well, to a concentration level expected for a well of the transistor. In an example for emulating an NMOS transistor, the doping of the well 121 would be P-type, as in the well 119 for the transistor. In a PMOS example, the well doping would be N-type, as in the well for the PMOS transistor.

The method of making the test structure also includes one or more steps S3 relating to forming a gate insulator layer 123 over the active device well (see FIG. 5), and forming a polysilcon layer 137 on the gate insulator layer 123 (see FIG. 8). The well may also be implanted with dopants to define a channel threshold (see FIG. 7). In the examples, the gate insulator layer 123 is applied prior to the initial doping of the well (see FIGS. 5 and 6). The polysilicon layer is applied after the well doping step (see FIG. 8).

In a subsequent processing step S4, dopant of a second semiconductor type is implanted in the polysilcon layer. In the test structure, the intent is for this implantation to achieve a concentration level corresponding to that expected in the transistor gate. In an embodiment where the test structure is made during the manufacture of the transistor, this part of the process involves an initial implantation performed for gates of both devices (FIG. 9). However, the gate of the transistor will be subject to doping during some later steps, which is blocked off from further implantation in the gate for the test structure. Hence, the example for making the test structure involves an extra implantation in the area of polysilicon for the test structure gate (FIG. 10), in order to bring the concentration up to the level expected in the transistor gate.

After dopant implantation in the polysilicon layer 137, the method (FIG. 3) involves a step S5 for patterning the polysilcon layer 137 and typically the gate insulator layer 123, to form a gate for the test structure (see also FIG. 11). At least one dimension of the gate of the test structure substantially corresponds to an expected dimension of the gate of the transistor. In examples of structures used to test parameters for the transistor of interest, the width and length of the gate for the test structure are approximately the same as in the transistor of interest for testing purposes. In a test structure used for calibration, e.g. for a gate length measurement application, the test structure may be a larger area device, but typically one or more dimensions (e.g. width and height) will still closely correspond to the gate of the transistor.

The method of making the test structure also includes a step S6 (FIG. 3) for forming insulating spacers on sidewall surfaces of the gate for the test structure (see FIG. 13). Dopant of the first semiconductor type is implanted into the active device well, to form

regions

185 and 187 in the test structure (see FIG. 15). These regions correspond to the source 173 and drain 175 of the transistor of interest (see FIG. 14). However, the regions in the test structure are doped with opposite type dopant material. The

regions

185 and 187 in the test structure, however, are implanted to a dopant concentration level that is substantially equal to a concentration of dopant of the second semiconductor type expected in source and drain

regions

173, 175 of the transistor.

In the example, the steps for making the test structure are incorporated into and rely on many steps involved in the making of the NMOS device of interest. Although not discussed in detail, the process of manufacturing a complementary metal oxide semiconductor (CMOS) integrated circuit will also include steps for making the complementary devices, in this case PMOS devices and possibly one or more test structures for characterizing the PMOS devices. It is advantageous that the production of the test structure(s) can be incorporated into the conventional process flow for making CMOS integrated circuits, with minimal additional steps relating to formation of the test structures.

It may be helpful now to consider a process for concurrent production of a transistor and its corresponding test structure in somewhat more detail. The exemplary process flow is illustrated in FIGS. 4–20.

FIG. 4 illustrates a basic SOI structure in its early stages of fabrication. The structure includes the silicon substrate 111, the silicon oxide insulator layer (referred to as a ‘box’) 113 and the top silicon layer 115. Isolation regions 117 separate the active device regions of the top silicon layer. For purposes of discussion of one example, it will be assumed that the method produces one NMOS type transistor device 211 and one test structure 213, as shown for example in FIG. 20. To produce those two devices, the basic SOI structure of FIG. 4 includes two active devices regions or

wells

119 and 121 separated from each other and from other active regions by the isolation regions 117. In an actual production device, there would be many active regions 119 for construction of many NMOS devices and a small number of active regions 121 for construction of test structures. There may also be many active regions for PMOS type devices as well as a small number of corresponding regions for one or more of the test structures used for analysis relating to the PMOS devices.

The basic structure of FIG. 4 may be formed via a SIMOX (Separation by Implantation of Oxygen) process. The SIMOX process involves implanting oxygen beneath the surface of a silicon wafer. An annealing step serves to coalesce the implanted oxygen atoms into a uniform layer 113 of SiO₂. Sometimes, epitaxial silicon may be grown atop the silicon to satisfy specific device requirements, but with or without an epitaxial layer, the top surface film 115 becomes the active structure for device fabrication. The buried oxide layer 113 is typically 0.1 to 0.5 μm thick and exhibits almost complete incorporation of the implanted oxygen. Typical implant energies range from 150 to 200 keV, while the oxygen dose may vary from 1 to 2E18/cm². The thickness of top silicon film 115 as well as the variation thereof with respect to the thickness of the oxide layer 113 are functions of the implant energy and the rate of surface silicon sputtering during the implant process. The SOI formation process may also involve high temperature annealing, for example, at temperatures greater than 1250° C. for several hours, in order to coalesce the implanted oxygen and achieve solid state recrystallization of the top (superficial) silicon layer 115 from the surface downward.

The isolation regions 117 may be formed by a variety of known techniques. For example, one technique involves masking and etching to form thin trenches between and around the

active regions

119 and 121. Following the formation of the trenches, a layer of oxide material is formed on the structure, for example, using high density plasma chemical vapor deposition (HDPCVD) to fill the trenches with the oxide material. Following deposition of the oxide material, the oxide material is polished via CMP, and any masking layers may be stripped by etching, to expose the upper surface level of

active device regions

119 and 121 of the semiconductor layer 115. The surfaces of the isolation regions 117 are substantially planar with the surface level of

active device regions

119 and 121.

FIG. 5 shows formation of a thin oxide layer 123, for use as the gate insulator(s). The gate oxide material 123 may be laid down on the top silicon layer 115 and the isolation regions 117 as shown, or the gate oxide material 123 may be laid down on top silicon layer 115 only in the active regions 119 and 121 (i.e. only between the isolation regions 117). The thin gate oxide material 123 may have a thickness in the range of about 40 Å or less. The thin gate oxide material 123 typically includes SiO₂, which has a substantially low dielectric constant, although any suitable insulating material (e.g., Si₃N₄) may be employed.

FIG. 6 illustrates implantation 125 of P-type dopants into the

active device wells

119 and 121 formed in the top silicon layer 115, so as to form the desired P-type active semiconductor regions. In the example, the bodies of the active regions will become P-

type semiconductor regions

127 and 129. The formation of the P-

type semiconductor regions

127 and 129 involves masking a portion of the top silicon layer 115 with a photoresist layer (not shown) formed on top of the gate oxide layer 123 and implanting P-well dopants to provide the P-type bodies, for example, with dopant concentrations of around 3–7×10¹⁷/cm³.

FIG. 7 represents the N-channel threshold implant step. This step involves implantation 131 of N-type dopants into the P-

type semiconductor regions

127 and 129. This implantation step forms lightly doped N-

type regions

133 and 135 along the surfaces of the P-

type semiconductor regions

127 and 129, respectively. This step typically involves masking a portion of the top silicon layer 115 with a photoresist layer (not shown) formed on top of the gate oxide layer 123. The mask may be the same as that used in the preceding step of implanting P-type dopants into the wells. When so masked, the step of FIG. 8 entails implanting N-type dopants at 131, to provide the N-type

channel threshold regions

133 and 135, for example, with dopant concentrations of around 2–100 keV, B11 or BF2 for nmos. The

channel threshold implants

133 and 135 will typically extend into the

semiconductor regions

127 and 129 to a desired depth below the surface of the top silicon layer 115.

The steps represented by FIGS. 8–11 relate to formation of the gates of the NMOS device and the test structure. As shown in FIG. 8, a polysilicon layer 137 is deposited on top of the gate insulator layer formed by the thin gate material 123. The polysilicon layer 137 may have a thickness within the range of about 1000 to 2000 Å, although the thickness of the layer 137 should be chosen to account for any subsequent polishing that might be performed to form the various gates. The polysilicon layer 137 may be formed by chemical vapor deposition (CVD), although any type of film formation process may be used.

Once formed, the polysilicon film 137 is pre-doped via, for example, ion implantation 139 in order to lower the resistivity of the polysilicon, as illustrated in FIG. 9. In the example, this implantation step 139 effects an N+ pre-doping. The pre-dopant concentration of the polysilicon film 137 may be in a range around 10²⁰/cm³. In this example, the N+ pre-doping is followed by Rapid Thermal Annealing, represented as part of FIG. 9. This annealing may be performed immediately after the doping of the polysilicon as shown, or it may be performed later after other implantation steps. For example, the range for implant energy may be 3–50 keV, and the type of dopant may be Phos. or Arsenic at concentration in a range of 1e19–1e21.

To this point in the process, the standard NMOS device and the corresponding test structure have been treated equally. The dimensions and doping have been essentially identical. However, some of the subsequent processing differs between the test structure and the NMOS device that will be characterized by measurements on the test structure. Although the NMOS device and the test structure will differ in some regards, certain conditions on the test structure match or mirror the NMOS device, so that the test structure will have similar parameters to the NMOS device when both devices are complete.

For example, a number of subsequent N+ implantation steps to form elements of the NMOS device will implant additional N-type dopants into the gate of the NMOS device, further lowering the resistivity of the transistor gate. The gate of the test structure, however, will be masked during these subsequent doping steps (as described below) and will not receive additional N-type doping. It is desirable that the characteristics of the gate of the test structure correspond to those of the gate of the NMOS device. To compensate for the masking in subsequent doping steps, the illustrated method therefore includes an additional pre-dope step, applied only to the portion of the polysilicon layer 137 that will be used to form the gate of the test structure, as shown in FIG. 10.

A mask 141 is applied to the polysilicon layer 137 over the region for the NMOS device. The exposed portion of the polysilicon film 137 is again pre-doped via, for example, ion implantation as shown at 143 in FIG. 10. This further N+ pre-doping typically uses implant energies ranging from around 1 keV to approximately 20 keV, and the implant dose may vary from 1E15 to 1E16/cm². The dopant material may be arsenic, or the like. This additional processing of the polysilicon for the test structure could involve a further Rapid Thermal Annealing, at this time or as part of a later general annealing of the semiconductor device.

The next step involves patterning of the polysilicon and insulator layers to form the gate electrodes and the underlying gate insulators, that is to say, to form the structure as shown by way of example in FIG. 11. The gate and insulator formation, for example, may entail a standard dry anisotropic etch process, although other patterning techniques may be utilized. As a result of this processing, the illustrated SOI integrated circuit now includes a gate 145 having an N+ doping characteristic and an underlying gate insulator layer 147, for use in the NMOS device. The illustrated SOI integrated circuit also includes a gate 149 having an N+ doping characteristic and an underlying gate insulator layer 151, for use in the test structure. In this example, the height, width and length dimensions of the gate 149 and the gate insulator 151 for the test structure are substantially the same as those of the gate 145 and the gate insulator 147 for the NMOS transistor. However, in view of the extra pre-doping step 143 (FIG. 12), the N+ doping concentration in the gate 149 is higher than that in the gate 145, at this stage of the process.

FIG. 12 represents the N− dopant implantation to form the source/drain extensions for the normal NMOS device. During formation of the source/drain extensions, portions of the test structure are masked, to prevent implantation of the dopants and formation of extensions in the test structure. Hence, the processing represented by FIG. 13 involves forming a mask 153 over the polysilicon gate and the exposed portions of the active device well, for the test structure. Once the test structure portions are masked, the N− dopant implantation at 155 forms a lightly doped source extension 157 and a lightly doped drain extension 159. The implantation uses the polysilicon gate 145 as a mask to effectively self-align the

extension regions

157 and 159, as illustrated in FIG. 12. Those skilled in the art will recognize that other methods of forming the

extension regions

157 and 159 may be employed. The N− extensions may have dopant concentrations around 10¹⁸/cm³. After the formation of the extension regions, an anneal may be performed; alternatively, a single anneal may be performed later in the process after the formation of the source/drain regions. As a result, the

regions

157 and 159 will typically extend somewhat under the gate 145, creating gate overlaps.

As noted, during the formation of the N−extensions, a mask 153 prevents implantation of the N-type dopants in step 155 into elements of the test structure. As a result, the test structure will not include source/drain extensions within the active device well, and the gate is not further doped. Overlap between the gate and the source drain extensions in the transistor prevents scaling of gate current to gate or channel length, so since the test structure has no overlap (see FIG. 20), the test structure 213 will exhibit a substantially linear relationship of gate current to gate/channel length. A difference in gate length results in a corresponding difference in current.

After extension formation for the NMOS device (FIG. 12), the mask 153 is removed from the test structure. Dielectric or insulating spacers are formed along the sidewalls of the gates, as shown in FIG. 13. For the NMOS device, this step produces

sidewall spacers

161 and 163 on opposite sides of the gate 145 and the underlying gate insulator. For the test structure, this step produces

sidewall spacers

165 and 167 on opposite sides of the gate 149 and the underlying gate insulator. In this example, all of the sidewall spacers have similar dimensions and consist of the same insulating material.

The sidewall spacers 161, 163 165 and 167 may be formed, for example, by CVD oxide deposition followed by a directional etching such as, for example, reactive ion etching (RIE). The spacer material layer may comprise tetraethoxysilane (TEOS) oxide, silicon dioxide, silicon nitride or the like. Of course, other techniques or insulating materials may be employed to form the various sidewall spacers.

FIG. 14 represents the deep implantation of N+dopant to form the source/drain regions for the normal NMOS device. During formation of the source/drain regions, portions of the test structure are again masked, to prevent receipt of the dopants and thus prevent formation of N-type source/drain regions in the test structure, although the gate 145 of the NMOS transistor receives further N type doping. Hence, the processing represented by FIG. 14 involves forming a mask 169 over the polysilicon gate and the exposed portions of the active device well, for the test structure. Once the test structure portions are masked, the N+ deep doping at 171 forms a deeply doped source region 173 and a deeply doped drain region 175. Implant energy for example may be 3–50 keV, and the type of dopant may be Phos. or Arsenic at a concentration in the range of 1e19–1e21.

The

spacers

161 and 163 on the sidewalls of the gate 145 of the NMOS device act as masks over the source and drain extensions, to prevent further implantation of N-type dopants into the

extensions

157 and 159. Consequently, the

extensions

157 and 159 remain lightly doped (N− in the example) and have doping concentrations essentially as implanted in the earlier step (FIG. 12). During formation of the N+ source/drain regions 173 and 175 (FIG. 14), portions of the test structure are masked, to prevent receipt of the N+ dopants and formation of N type source drain regions in the test structure. After the formation of the source and drain regions, an anneal may be performed; alternatively, a single anneal may be performed later in the process after the formation of the corresponding regions in the test structure.

The mask 169 is removed from the test structure. In the example, the next step is to form P+ regions in the test device, substantially corresponding to source

drain regions

173 and 175 of the NMOS device. However, to prevent P-type doping of the gate 149, a mask 179 is formed on the gate to block P-type implantation. Also, a mask 181 is formed over the normal NMOS device to block introduction of P-type dopants into any of the elements of the NMOS device.

Before the p+ source and drain regions are formed, the mask(s) on the gate should be longer than the gate length. In the example, it is important for the p+ source and drain regions to NOT get into the gate and change the dopant profile. It is okay for the p+ source and drain regions to be placed away from the spacer if needed—in other words the mask could come all the way to the edge of the spacer. In general that would depend on the alignment tolerance for the technology.

At 183, the test structure is subjected to the P+ deep doping. This implantation step forms the P+ regions shown at 185 and 187 in FIG. 15. Although the highly doped (P+)

regions

185 and 187 do not actually function as a source and drain of a field effect transistor, since they are not N-type regions (do not form an NPN transistor with the channel), they correspond in size, shape and doping concentration to the source 173 and drain 175 of the corresponding NMOS transistor. Hence, for ease of discussion, the

P+ regions

185 and 187 of the test structure are referred to as the source and drain of the test structure.

The intent is for elements of the test structure to correspond closely to those of the NMOS transistor that the test structure will emulate. Although the dopant type differs, the dimensions and dopant concentrations of the source 185 and drain 187 of the test structure are substantially the same as those of the source 173 and drain 175 of the corresponding NMOS device. Another difference between the source and drain of the NMOS device and the test structure is that the NMOS device includes the

extensions

157 and 159, whereas the P+ source and drain in the exemplary test structure do not include any corresponding extensions.

After removal of the masks (FIG. 16), the NMOS transistor and the test structure are relatively complete. Processing could go forward with formation of sealing/insulators over both devices, vias through the insulators and contacts through the vias for electrical connections. However, in the example, a series of intermediate steps are performed to form silicide layers, for example in the gates and in the source and drain regions, of both the normal NMOS transistor device and the test structure, which facilitate the electrical connections.

As illustrated in FIG. 17, silicide formation is initiated by depositing a layer of metal 189 across the exposed surfaces of the semiconductor devices, including the

gates

145 and 149, and the various

source drain regions

173, 175, 185 and 187 (compare to FIG. 16). The metal layer 189 may be formed from a suitable metal, such as titanium, cobalt, or nickel. The metal layer 189 may be deposited, for example, by sputtering.

The desired silicide regions are then formed by reacting the metal layer 189 with the underlying surface portions of the

gates

145 and 149, and the various

source drain regions

173, 175, 185 and 187, that is to say, the regions of the NMOS device and the test structure in contact with the metal layer 189. This silicide formation step may utilize any one of a number of silicidation or salicidation processes. An exemplary technique employees thermal annealing, which essentially raises the temperature of the semiconductor devices to a suitable level (e.g., about 500° C. to about 700° C.) for a suitable length of time (e.g., about 10 seconds to about 10 minutes). Rapid thermal annealing (RTA) may also be employed, for example at a temperature of about 600° C. to about 900° C. for about 5 second to about 120 seconds. It will be appreciated that other temperatures and heating times may be employed.

The chemical reaction of the metal layer 189 with the exposed semiconductor elements forms a number of silicide regions. In the NMOS transistor device, the processing forms a silicide region 191 on the gate, as well as

silicide regions

193 and 195 on the source and drain of the NMOS device, as shown in FIG. 18. In the test structure, the processing forms a silicide region 197 on the gate, as well as

silicide regions

199 and 201 on the source and drain of the test structure, as further shown in the drawing. As illustrated, the

silicide regions

193, 195, 199 and 201 will tend to encroach underneath the respective sidewall spacers. In an example, these silicide regions will encroach under the sidewall spacers a lateral distance of about zero to about 100 Å.

Unreacted metal in the metal layer 189 is removed to expose the underlying layers. FIG. 19 shows the NMOS device and the test structure, after removal of the metal layer 189. The silicide layers are now exposed to allow formation of contacts (not shown), for purposes of making electrical circuit connection to the various device elements. The silicide layers reduce contact resistance of subsequent connections to the various regions of the test structure and the NMOS device.

FIG. 20 shows the NMOS device 211 and the test structure and illustrates a current flowing between the gate and one of the P+ source/drain regions of the test structure 213.

The resulting test structure 213 looks like an MOS type field effect transistor (MOSFET). The illustrated test structure 213 does not include the NPN (or PNP) type regions normally found in a transistor. Essentially, the structure comprises a gate, an insulator and a semiconductor well having a number of different dopant levels. As such, the test structure is a form of integrated circuit type capacitor. However, the characteristics of the test structure 213 are designed to enable testing thereof to provide an accurate indication of gate current as it would appear in a corresponding transistor device 211.

The areas, depths/heights and concentrations of many elements of the test structure 213 are substantially similar to those of corresponding elements 149 in the transistor 211 of interest. However, the test structure lacks halos or extensions in the source/drain regions. Hence the gate 149 of the test structure does not substantially overlap the source 185 or the drain 187. Also, the test structure has P+ regions for the source 185 and the drain 187, where the conventional NMOS transistor 211 has N+ regions for its source 173 and drain 175. The elimination of halos and/or extensions and the use of dopants of the same type as the body, enable testing of the structure 211 to accurately measure only the intrinsic gate-body current that would normally appear in the similar NMOS transistor of interest. Also, the P-body formed within the semiconductor region 129 is well contacted by the low resistance P+ source or drain region (185 or 187). In a calibration type test device, the gate has a known wide area. In a test device corresponding to an actual transistor, the gate width and length are substantially the same as for the gate on the transistor of interest.

In each

active semiconductor region

127 or 129, the portion thereof other than the source/

drain regions

173, 175 or 185, 187 may be considered as the semiconductor body. In an SOI transistor, such as NMOS device 211, the body floats relative to ground in that there is no specific potential or voltage value applied to the body of such a transistor. In fact, a normal operational SOI transistor does not have any electrical connection to the body portion of the device. The P+ ‘source/drain’

emulation regions

185, 187 in the test structure 213, however, provide a means to electrically connect to the body in the test structure, for example, to measure current flow through the body as discussed above relative to FIGS. 1 and 2.

In the production methodology outlined above, the test structure 213 was carefully constructed to match one or more regular NMOS devices 211 (often there are many similar NMOS devices 211 in the same integrated circuit). For example, the dimensions of the active well, the gate, channel and source drain of the test structure were the same as those of the regular MNOS devices 211, the dopants in the channel implant and the gate were the same in the test structure 213 and the MNOS device 211, and the P-type dopants in the source region 185 and the drain region 187 have concentration levels that correspond to those of the N-type dopants in the source region 173 and the drain region 175 of the NMOS device 211. As such, measurements taken on the test structure 211 fairly characterize properties of the corresponding NMOS device 213. However, the SOI integrated circuit may include one or more implementations of the test structure that do not correspond in size to a particular NMOS device 211. For example, the SOI integrated circuit may include a test structure produced by a similar technique but having “large area,” for calibration purposes.

The examples described in detail above focused on a test structure designed to mimic an NMOS transistor. For example, the gate is doped with N+ materials, and the regions corresponding to the source and drain are P+ regions. Those skilled in the art will appreciate, however, that the test structure and its manufacturing technique are equally adaptable to applications for testing in relation to a PMOS transistor (instead of the NMOS device). Essentially, the manufacturing processing is the same, except that the types of the dopants are reversed. Where P type dopants are used to build the test structure 213 for analysis of the NMOS type transistor 211 (the illustrated example), manufacture of a similar test structure for analysis of a PMOS type transistor would utilize N type dopants. Conversely, where N type dopants are used to build the NMOS test structure 213, manufacture of a similar test structure for PMOS would utilize P type dopants.

Also, the measurements on the test structure (FIGS. 1 and 2) focused on applications to SOI type structures, e.g. gate body current which is relevant to floating body effects. However, those skilled in the art will recognize that the test structure and its method of manufacture may be adapted for use with bulk silicon-based MOS devices, for example, to measure the physical gate length of a bulk MOS device of interest.

While the foregoing has described what are considered to be the best mode and/or other examples, it is understood that various modifications may be made therein and that the invention or inventions disclosed herein may be implemented in various forms and examples, and that they may be applied in numerous applications, only some of which have been described herein. It is intended by the following claims to claim any and all modifications and variations that fall within the true scope of the inventive concepts.

Claims

1. An integrated circuit comprising a metal oxide semiconductor transistor, and an associated test structure for testing to analyze one or more characteristics of the metal oxide semiconductor transistor,

the metal oxide semiconductor transistor comprising:

a channel of a first semiconductor type,

a drain of a second semiconductor type adjacent to a first side of the channel,

a source of the second semiconductor type adjacent to a second side of the channel opposite the first side of the channel,

an insulator over the channel, and

a gate on the insulator; and

the test structure comprising:

a test structure channel of the first semiconductor type,

a test structure drain of the first semiconductor type adjacent to a first side of the test structure channel,

a test structure source of the first semiconductor type adjacent to a second side of the test structure channel opposite the first side of the test structure channel,

an insulator over the test structure channel, and

a test structure gate on the test structure insulator,

wherein a test parameter measurable on the test structure closely corresponds to a parameter of the metal oxide semiconductor transistor.

2. The integrated circuit as in claim 1, wherein:

the source of the metal oxide semiconductor transistor comprises: a deeply doped region, and a lightly doped region extending under the gate of the metal oxide semiconductor transistor; and

the drain of the metal oxide semiconductor transistor comprises: a deeply doped region, and a lightly doped region extending under the gate of the metal oxide semiconductor transistor; and

the test structure gate is substantially free of overlap with respect to the test structure source and the test structure drain.

3. The integrated circuit as in claim 1, wherein the test parameter measurable on the test structure comprises gate-body current.

4. The integrated circuit as in claim 1, wherein dopant concentrations in the test structure source and the test structure drain are substantially equal to dopant concentrations in the source and the drain of the metal oxide semiconductor transistor.

5. The integrated circuit as in claim 1, wherein length of the test structure gate over the test structure channel is substantially equal to length of the gate of the metal oxide semiconductor transistor.

6. The integrated circuit as in claim 5, wherein resistivity of the test structure gate is substantially equal to resistivity of the gate of the metal oxide semiconductor transistor.

7. The integrated circuit as in claim 5, wherein:

the metal oxide semiconductor transistor further comprises a channel threshold implant of the second semiconductor type; and

the test structure further comprises a channel threshold implant of the second semiconductor type.